The three main UK networks are O2, 3 and Orange.
See the links for their prices.
There are a lot of variations, but the best value ones are probably the 'SIM Only' schemes if you already have an unlocked GSM phone. Some of these are only a one month contract, so you can change any time you like.
They all include caller ID and most have unlimited texts (or many thousands).
Minutes are only outgoing calls. Some give free calls to other users on the same network.
Some plans have unlimited calls to UK land lines either evenings or all the time.
(Note that this is only to normal phone numbers with a geographic area code. 08 & 09 numbers are special rate codes).
The bit that adds cost is the 500MB Data, that probably means getting to the 'Smartphone' price level rather than normal voice plans.
If you need a new smartphone as well, you will have to get a 12/18/24 month contract to cover the cost of the phone.
Whatever you get, buy it online rather than in a shop!
The UK / Europe has very strict consumer protection law, and one part of that is the 'Distance Selling Regulations' - if you buy any normal item online, you have the option to tell the seller you don't want it within eight days of receiving it, and getting a full refund of what you paid.
That does not apply in shops, once you sign the paperwork you are stuck with it.
You get protection from the Sale of Goods act regs regardless. Under English law, all warranties, faults, problems are the responsibility of the Seller, not the maker, and generally for one year.
Also, if you get a contract phone from O2 online, they seem to send unbranded and unlocked phones - I've had several over the years & none have been locked. All the re-sellers contract phones are network locked.
Many big named chains have completely free WiFi access for customers - McDonald, Weatherspoons pubs, Starbucks etc.
There are also chargeable public WiFi access points in many places - you pay eg. £5 for a days use. Some also have other plans or agreements with mobile providers to give free or cheap access to their users.
BT Openzone is one of these.
For electronics & computer parts, again online is probably cheapest.
A couple of big chains are Maplins for electronics, CCTV etc., PC World / Currys for computers.
Maplins are generally OK.
PC World are not bad for actual computers, printers etc. but accessory prices are crazy - eg. £10 to £20 for an ethernet cable you get for pence online.
There are lots of small independent computer & electronics shops, they are generally reasonable prices. Also, many bigger Supermarkets have reasonable electronics departments.
You can even get quite a range of common audio / video / PC cables from some 'Pound Shops'.